Fatigue Behavior Of Alloy Research Articles

The influence of pre-strain levels on the microstructure and performance of aircraft 7b04 aluminum alloy skin

The study focuses on the 7B04 alloy sheet. It investigates the effects of different pre-strain levels on the microstructure, mechanical properties, and fatigue behavior of the alloy through the use of optical microscopy, scanning electron microscopy, and DIC (Digital Image Correlation) system. The experimental results demonstrate that the yield strength and tensile strength of the alloy increase with the increase in pre-strain levels. At the same time, the elongation decreases with the increase in tensile deformation. In the non-uniform region, the plastic deformation capacity index of the material (denoted as ψ) follows the order ψ (non-uniform region (DIC value)) > ψ (original gauge length value) > ψ (uniform region (DIC value)). It is recommended to control the maximum local strain below ψ (uniform region (DIC value)) during production to avoid deformation instability or concentration. Furthermore, the thickness of the material decreases linearly with increasing tensile deformation, with each 4% increase in tensile deformation resulting in a 1.2% reduction in thickness. The fatigue life of the alloy increases gradually with the increase in pre-strain levels in the direction perpendicular to the fiber. At the same time, it decreases in the direction parallel to the fiber. Different deformation levels and loading directions exhibit similar fatigue fracture characteristics. Within the range of 12% pre-strain levels, the microstructure of the material remains continuous and intact without noticeable cracks at grain boundaries or phase boundaries. At the macroscopic level, when the pre-strain level exceeds 6%, distinct slip bands appear on the sheet, which is correlated with the micro-scale grain slip. This is accompanied by deformation bands and micro-cracks in the microstructure. The findings of this study are of significant importance for understanding the influence of pre-strain levels on the mechanical properties and fatigue behavior of the 7B04 alloy. The study provides valuable references for engineering practices in related fields, thereby ensuring the integrity and usability of manufactured components.

Thermally activated dependence of fatigue behaviour of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion

The CrMnFeCoNi high-entropy alloy demonstrates a promising potential for applications over a range of temperature. The alloy also shows excellent printability to be fabricated by additive manufacturing for complex structures. Nevertheless, there are limited studies on the thermo-mechanical behaviour of the alloy, in particular when fabricated by laser powder-bed fusion. This study provides an in-depth understanding of the relationship between as-built cellular microstructures and fatigue behaviour at a range of temperatures (22–600 °C) in particular concerning the stability of dislocation cells and thermo-mechanical dependence of the fatigue behaviour of the alloy. At all tested temperatures, the alloy exhibits a very short duration cyclic hardening with a low hardening rate followed by a cyclic softening. The high density of dislocations already existing in as-built condition were able to accommodate most of the prescribed strain. Hence, only a small number of mobile dislocations needs to be generated, causing a short cyclic hardening phase. Upon further loading, the back stress associated with the long-range stress field was dominant factor governing the cyclic softening behaviour. The similitude relationship provided insights into the stability of as-built cells, in particular it explains why the size of as-built cells did not change during cyclic loading at 22 °C. The significant reduction in dislocation density due to the increased annihilation rate and untanglement of dislocation substructures thanks mainly to thermal assistance at elevated temperatures led to a decrease in cyclic strength and related properties (yield stress, friction and back stress, hysteresis loop shape parameter and energy per cycle). The LPBF HEA shows an insignificant strain rate dependence of the primary cyclic hardening and softening in the range of 10 −3 s −1 and 10 −2 s −1 . However, the dynamic strain ageing results in a secondary cyclic hardening at 400 °C and the reversed strain sensitivity at temperatures from 200° to 400 °C. The fracture mode was transgranular at 22–400 °C but changed to more intergranular-like at 600 °C due to the decohesion of grain boundaries, resulting in a reduction in fatigue life.

Effects of welding columnar grain orientation and strain rate on corrosion fatigue behavior of Alloy 52/52M weld metal in high-temperature water

Corrosion fatigue (CF) tests of Alloy 52/52 M weld metal from a safe-end welding joint of reactor pressure vessel were performed in B/Li high-temperature water. The CF cracks developed preferentially in 52 Mw with columnar grain orientation at a 45° - 75° angle to cyclic loading axis, rather than in 52b with columnar grain orientation parallel to loading axis. The environmental assisted fatigue effects were significant under lower strain amplitudes and the effect of strain rate on CF behavior was not significant. The CF cracking mechanism involved with welding columnar grain orientation is discussed.

Effects of intermittent loading time and stress ratio on dwell fatigue behavior of titanium alloy Ti-6Al-4V ELI used in deep-sea submersibles

Different components of deep-sea submersibles, such as the pressure hull, are usually subjected to intermittent loading, dwell loading, and unloading during service. Therefore, for the design and reliability assessment of structural parts under dwell fatigue loading, understanding the effects of intermittent loading time on dwell fatigue behavior of the alloys is essential. In this study, the effects of the intermittent loading time and stress ratio on dwell fatigue behavior of the titanium alloy Ti-6Al-4V ELI were investigated. Results suggest that the dwell fatigue failure modes of Ti-6Al-4V ELI can be classified into three types, i.e., fatigue failure mode, ductile failure mode, and mixed failure mode. The intermittent loading time does not affect the dwell fatigue behavior, whereas the stress ratio significantly affects the dwell fatigue life and dwell fatigue mechanism. The dwell fatigue life increases with an increase in the stress ratio for the same maximum stress, and specimens with a negative stress ratio tend to undergo ductile failure. The mechanism of dwell fatigue of titanium alloys is attribute to an increase in the plastic strain caused by the part of the dwell loading, thereby resulting in an increase in the actual stress of the specimens during the subsequent loading cycles and aiding the growth of the formed crack or damage, along with the local plastic strain or damage induced by the part of the fatigue load promoting the cumulative plastic strain during the dwell fatigue process. The interaction between dwell loading and fatigue loading accelerates specimen failure, in contrast to the case for individual creep or fatigue loading alone. The dwell fatigue life and cumulative maximum strain during the first loading cycle could be correlated by a linear relationship on the log–log scale. This relationship can be used to evaluate the dwell fatigue life of Ti alloys with the maximum stress dwell.

On the Microstructure of Laser Beam Powder Bed Fusion Alloy 718 and Its Influence on the Low Cycle Fatigue Behaviour.

Additive manufacturing of Alloy 718 has become a popular subject of research in recent years. Understanding the process-microstructure-property relationship of additively manufactured Alloy 718 is crucial for maturing the technology to manufacture critical components. Fatigue behaviour is a key mechanical property that is required in applications such as gas turbines. Therefore, in the present work, low cycle fatigue behaviour of Alloy 718 manufactured by laser beam powder bed fusion process has been investigated. The material was tested in as-built condition as well as after two different thermal post-treatments. Three orientations with respect to the building direction were tested to evaluate the anisotropy. Testing was performed at room temperature under controlled amplitudes of strain. It was found that defects, inclusions, strengthening precipitates, and Young’s modulus influence the fatigue behaviour under strain-controlled conditions. The strengthening precipitates affected the deformation mechanism as well as the cycle-dependent hardening/softening behaviour. The defects and the inclusions had a detrimental effect on fatigue life. The presence of Laves phase in LB-PBF Alloy 718 did not have a detrimental effect on fatigue life. Young’s modulus was anisotropic and it contributed to the anisotropy in strain-life relationship. Pseudo-elastic stress vs. fatigue life approach could be used to handle the modulus-induced anisotropy in the strain-life relationship.

The fatigue crack behavior of 7N01-T6 aluminum alloy in different particle environments

An experimental method of evaluating the fatigue behavior of alloys in different particle environments was designed, and the effects of four kinds of particles (i.e., graphite, CaO, Al2O3, and MnO2) on the crack propagating behavior of 7N01-T6 behaviour alloys were investigated. The results show that the particles deposited on the crack surface exert significant influence on the fatigue crack propagation behavior thereof. This influence strongly depends on the elastic moduli of the particles (Ep). As Ep is less than that of aluminium alloy (EAl), the particle accelerates the fatigue-crack-growth rate (FCGR) in the alloys due to the lubrication of the particles on the mating fracture surfaces. When the difference between Ep and EAl is small, the particle effect on the FCGRs of the alloys is small due to the counteraction between the decrease in friction and the promotion on the crack closure of mating fracture surfaces. When Ep is greater than EAl, the particles slow down the FCGRs of the alloys on account of significant crack closure effect. As Ep is much greater than EAl, the particles increase the FCGRs because of the increasing stress concentration at the crack tip.

Role of welding residual strain and ductility dip cracking on corrosion fatigue behavior of Alloy 52/52M dissimilar metal weld in borated and lithiated high-temperature water

Corrosion fatigue behavior of Alloy 52/52M dissimilar metal weld (DMW) was investigated in borated and lithiated high-temperature water. The fatigue life of Alloy 52/52M DMW in high-temperature water decreased in comparison with that in air. The fatigue cracks initiated at Alloy 52M butt weld or at the interface of Alloy 52 buttering and Alloy 52M butt weld. The welding residual strain and ductility dip cracking in Alloy 52M butt weld promoted the initiation and propagation of fatigue cracks. The environmentally assisted fatigue damage in high-temperature water is discussed by taking account of microstructure defects.

Evaluation of high cycle fatigue behaviour of alloy 617M at 973 K: Haigh diagram and associated mechanisms

Combined influence of mean stress and alternating stress on the high cycle fatigue (HCF) behaviour of Alloy 617 M at 973 K is discussed in the present study. Constant life Haigh diagrams have been generated using different combinations of alternating stress (σa) and mean stress (σm). The alloy exhibited dynamic strain aging (DSA) which is found to strongly influence the cyclic life as well as the fatigue limit. Apart from DSA, presence of annealing twins and carbide precipitation at high temperature during fatigue deformation is also found to positively influence the fatigue life of the alloy.

Effects of normal load on fretting corrosion fatigue of Alloy 690 in 285 °C pure water

Fretting corrosion fatigue behaviour of Alloy 690 was investigated in 285 °C pure water under different normal loads. With increasing the normal load applied, the fatigue life decreased, more severe surface damage and subsurface deformation were observed. The effects of normal loads on crack characteristics on the longitudinal sections across the fretting fatigue scars and corrosion fatigue fractures were examined. Possible influence of the normal loads on initiation and propagation mechanisms of the fretting corrosion fatigue cracks of Alloy 690 in high-temperature pure water are discussed.

Fretting corrosion fatigue of Alloy 690 in high-temperature pure water

Fretting fatigue behavior of Alloy 690 in room-temperature (RT) air and high-temperature pure water was investigated under the condition of a normal load of 100 N. Fretting fatigue life in high-temperature water reduced by about 30% compared to that in RT air due to combined effects of high-temperature water and highly localized tangential stress. The main fatigue cracks were inclined to initiate in the crevice region along the contact edges in high-temperature water, but none was observed in RT air. Related mechanisms of fretting corrosion fatigue failure of Alloy 690 in high-temperature water are discussed.

Fatigue behavior of a Fe-Mn-Si shape memory alloy used for prestressed strengthening

Cyclic deformation and fatigue behavior of an iron-based shape memory alloy (Fe-SMA) Fe-17Mn-5Si-10Cr-4Ni-1(V,C) were studied. In the first step, cyclic tensile tests were performed to characterize the material's mechanical properties in tension (elongation at break, yield, and tensile strength) as well as the recovery behavior of the alloy. Furthermore, the effect of strain rate on the cyclic loading tests was investigated. It was observed that the strain rate has a clear influence on the stress-strain behavior of the alloy. In the second step, the fatigue behavior of the alloy after pre-straining and thermal activation under strain-controlled conditions was evaluated. While the stiffness of the alloy remained almost constant during high-cycle fatigue loading, a decrease in the recovery stress was observed, which should be taken into account in design assessments. The loss in the recovery stress was assumed to be mainly a result of a transformation-induced relaxation (TIR) under cyclic loading. Furthermore, this study examines the applicability of a constant life diagram (CLD) model to determine the fatigue limit of the alloy for different stress ratios (R). The existing results of the fatigue tests showed full consistency with the proposed fatigue design criterion. A formulation based on the CLD model was proposed for a safe design of the alloy as a structural pre-stressing element under a high-cycle fatigue loading regime.

Corrosion Fatigue Crack Growth Behavior of Alloy 690 in High-Temperature Pressurized Water

Corrosion fatigue crack growth behavior of Alloy 690 was investigated in air and high-temperature pressurized water environments. The stress intensity factor amplitude threshold (ΔKth) was obtained, and the effects of stress intensity factor (ΔK), cyclic frequency (f), loading ratio (R), and material orientation on the corrosion fatigue crack growth rates were studied. The observed environmental acceleration on corrosion fatigue crack growth was quantitatively analyzed, and an empirical formula and the Ford-Andresen model were utilized to describe and explain the corrosion fatigue behavior of Alloy 690.

Understanding Low Cycle Fatigue Behavior of Alloy 617 Base Metal and Weldments at 900 °C

In order to better understand the high temperature low cycle fatigue behavior of Alloy 617 weldments, this work focuses on the comparative study of the low cycle fatigue behavior of Alloy 617 base metal and weldments, made from automated gas tungsten arc welding with Alloy 617 filler wire. Low cycle fatigue tests were carried out by a series of fully reversed strain-controls (strain ratio, Rε = −1), i.e., 0.6%, 0.9%, 1.2% and 1.5% at a high temperature of 900 °C and a constant strain rate of 10−3/s. At all the testing conditions, the weldment specimens showed lower fatigue lives compared with the base metal due to their microstructural heterogeneities. The effect of very high temperature deformation behavior regarding cyclic stress response varied as a complex function of material property and total strain range. The Alloy 617 base weldments showed some cyclic hardening as a function of total strain range. However, the Alloy 617 base metal showed some cyclic softening induced by solute drag creep during low cycle fatigue. An analysis of the low cycle fatigue data based on a Coffin-Manson relationship was carried out. Fracture surface characterizations were performed on selected fractured specimens using standard metallographic techniques.

Environmentally Assisted Fatigue Evaluation Model of Alloy 690 Steam Generator Tube in High-Temperature Water

Based on corrosion fatigue tests using a boat-shaped specimen in borated and lithiated high-temperature water, the corrosion fatigue behavior of Alloy 690 tubes was investigated. An Institute of Metal Research fatigue model for the nickel-based alloys is proposed, which incorporates fatigue life data of Alloy 690 tubes, as well as round-bar specimen data from the open literature. The proposed environmental fatigue life correction factor Fen addresses the effects of temperature, strain rate, and dissolved oxygen concentration. An evaluation methodology for environmental fatigue damage of structural materials in nuclear power plants is proposed.

Low Cycle Fatigue Behaviors of Alloy 617 (INCONEL 617) Weldments for High Temperature Applications

In this study, we comparatively investigate the low cycle fatigue behavior of Alloy 617 (INCONEL 617) weldments by gas tungsten arc welding process at room temperature and 800 °C in the air to support the qualification in high temperature applications of the Next Generation-IV Nuclear Plant. Axial total-strain controlled tests have been performed with the magnitude of strain ranges with a constant strain ratio (Rε = −1). The results of fatigue tests consistently show lower fatigue life with an increase in total strain range and temperature at all testing conditions. The reduction in fatigue life may result from the higher cyclic plastic strain accumulation and the material ductility at high temperature conditions. A constitutive behavior of high temperature by some cyclic hardening was observed. The occurrence of serrated yielding in the cyclic stress response was also observed, suggesting the influence of dynamic strain aging during high temperature. We evaluated a well-known life prediction model through the Coffin-Manson relationship. The results are well matched with the experimental data. In addition, low cycle fatigue cracking occurred in the weld metal region and initiated transgranularly at the free surface.

The effect of dissolved oxygen on fatigue behavior of Alloy 690 steam generator tubes in borated and lithiated high temperature water

Corrosion fatigue behavior of Alloy 690 tubes used for actual steam generator (SG) was investigated in borated and lithiated high temperature water containing <5ppb (by weight) and 5500ppb dissolved oxygen (DO) using boat-shaped specimens. It was found that the fatigue life at 5500ppb DO was longer than at <5ppb DO. The surface cracks and fracture characteristics of the specimens after fatigue tests were carefully examined. The effects of DO concentration on fatigue crack initiation and propagation of Alloy 690 tubes in high temperature water are discussed.

Corrosion fatigue behavior of Alloy 690 steam generator tube in borated and lithiated high temperature water

Corrosion fatigue behavior of Alloy 690 tube used for actual steam generator (SG) was investigated in <5ppb (by weight) dissolved oxygen borated and lithiated high temperature water by low cycle fatigue tests with boat-shaped specimens. It was found that the present tube materials had longer fatigue life than round-bar materials in open literature. The surface cracks were tortuous, typical crack branching and linkage were observed. The fracture surfaces were rough with well-defined fatigue striations. Related mechanisms of fatigue crack initiation and propagation of Alloy 690 SG tube in high temperature water are discussed.

Role of TiN inclusion on corrosion fatigue behavior of Alloy 690 steam generator tubes in borated and lithiated high temperature water

Influence of TiN inclusion on corrosion fatigue behavior of three types of as-received Alloy 690 steam generator tubes (J, C1 and C2) in borated and lithiated high temperature water was investigated. The volume fraction of inclusions in tube J, C1 and C2 were about 0.0894%, 0.0384% and 0.1843%. The fatigue life of tube C2 was slightly shorter than that of tube J and C1. Large broken TiN inclusions and TiN-inclusion clusters played a significant role on fatigue crack initiation and propagation for tube C2 in high temperature water. The inclusion-involved fatigue cracking mechanisms are discussed.

Effects of crack tip plastic zone on corrosion fatigue cracking of alloy 690(TT) in pressurized water reactor environments

Alloy 690(TT) is widely used for steam generator tubes in pressurized water reactor (PWR), where it is susceptible to corrosion fatigue. In this study, the corrosion fatigue behavior of Alloy 690(TT) in simulated PWR environments was investigated. The microstructure of the plastic zone near the crack tip was investigated and labyrinth structures were observed. The relationship between the crack tip plastic zone and fatigue crack growth rates and the environment factor Fen was illuminated.

Low cycle fatigue behavior of alloy 690 in simulated PWR water—Effects of dynamic strain aging and hydrogen

Low cycle fatigue (LCF) tests of Alloy 690 were performed in air and simulated PWR water under various loading conditions, and the results were analyzed to understand the LCF behavior of Alloy 690 in PWR water. The LCF life of Alloy 690 was shorter in PWR water than in air; however, the degree of reduction was less significant than for austenitic SSs. Through the tests and subsequent analysis, it was observed that dynamic strain aging (DSA) and hydrogen evolution by metal dissolution near the crack tip affect the fatigue crack growth behavior in PWR water. While DSA was observed, its effect on the LCF life was not evident. A hydrogen-induced dislocation structure was observed near the crack surface in TEM micrographs, indicating the operation of hydrogen-induced cracking (HIC) in PWR water. However, due to improved corrosion resistance, HIC was less operative for Alloy 690 than for austenitic SSs during the LCF test in PWR water. Consequently, the LCF life of Alloy 690 was longer than that of austenitic SSs in PWR water.